Most strains of Clostridium Perfringens isolated from their normal habitats possess a serologically and chemically distinct capsular material that covers the cell surface masking the common somatic antigens. During an investigation of the chemical and immunochemical properties of the specific capsular polysaccharide from several strains of C. perfringens it was observed that antisera prepared against Hobbs 5 crossreacted with hot water extracts from C. perfringens Hobbs 1, 9, 10 by immunodiffusion (ID). The crossreactive material was removed from the specific substance of Hobbs 5 and Hobbs 10 by ion exchange chromatography and gel permeation chromatography respectively. The activity in immunodiffusion was not affected by heating the antigen at 100 degrees, proteolytic digestion, or NaIO4 oxidation. The common antigen did not form a precipitin line by ID with immune serum prepared against the other strains. It is our intention to purify the common antigen by physical-chemical and chromatographic technique using ID as an analytical tool to follow its purification. The chemical properties indicate that the common antigen is a polysaccharide or glycopeptide. Therefore we would also follow the chemical composition of the various fractions by wet chemical techniques, paper chromatography and gas- liquid chromatography of hydrolyzed fractions. The isolation of a common antigen may enable the future development of a monovalent reagent for the identification of C. perfringens Type A. The relationship of the common antigen to an identical or similar polymers in Clostridium perfringens Type B-E will be investigated.